Heat oven to 350. Butter and flour the pan.
Follow all directions on the box, regardless of pan size.

While brownies are baking, unwrap all of the peppermint patties.
When there are 2 minutes left to bake, open the oven and evenly spread the peppermint patties on the brownies. Bake for 1 1/2 more minutes.

Immediately after taking brownies out of the oven, spread the peppermint patties around using a metal spatula. Let cool for at least 1/2 hour to let the peppermint and chocolate separate.